1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet to an image forming apparatus, etc.
2. Related Background Art
FIG. 13 schematically shows the sectional construction of a sheet feeding apparatus 300 arranged in an image forming apparatus as a prior art. In FIG. 13, reference numerals 301, 302 and 303 respectively designate a sheet feeding tray receiving (storing) sheets, a middle plate, and a coil spring as a biasing means for biasing the middle plate 302 upward. The middle plate 302 is axially supported by the sheet feeding tray 301 and can stack paper sheets of all sizes which is available by the image forming apparatus.
Reference numerals 304 and 305 respectively designate a separating pad for preventing double feeding of sheets, and a sheet feeding roller. The sheet feeding roller 305 is formed in a fan shape for feeding (sending) out one sheet on the uppermost face of a sheet bundle stacked on the middle plate 302 by frictional force rotated by an unillustrated controlling means.
When a small diameter portion of the fan shape of the sheet feeding roller 305 is opposed to the separating pad 304, a sheet feeding roller 306 separates the separating pad 304 and the sheet feeding roller 305 from each other and is rotatably held so that the sheet feeding roller 306 is rotated in accordance with a movement of the sheet.
The sheet fed from the sheet feeding roller 305 is conveyed by conveying roller pairs 307, 308 arranged in a sheet conveying path 309. These conveying roller pairs 307, 308 further apply conveying force to the fed sheet and convey the sheet even when the rotation of the sheet feeding roller 305 is stopped. The conveying rollers 307 and 308 are respectively a driving roller rotated by driving force, and a conveying roller biased against the conveying roller 307 by an unillustrated biasing means and rotatable around a predetermined axis as a center. Reference numeral 310 designates a feeder frame for holding the sheet feeding roller 305, etc., and attachably and detachably supporting the sheet feeding tray 301.
Reference numeral 311 designates a presence/absence sensor flag for detecting the presence/absence of sheets on the middle plate 302. Presence/absence information of the sheet can be inputted by this presence/absence sensor flag 311 to the image forming apparatus by switching operating states of an unillustrated detecting means. Reference numerals 312 and 313 respectively designate a rear end regulating plate and a side end regulating plate for determining a stacking position of the sheets stacked on the middle plate 302.
However, in the above conventional example, the middle plate 302 has a rotatable one-plate structure supported by a supporting shaft so that the following problems exist.
(1) A weight of the sheets stacked onto the middle plate 302 is greatly changed in accordance with a sheet size. Therefore, force (=sheet feeding pressure) for pressing a sheet by the biasing means for biasing the middle plate 302 against the sheet feeding roller 305 varies, so that the sheet feeding pressure is changed in accordance with the sheet size. Accordingly, it was difficult to stably feed the sheet in accordance with various sheet sizes.
(2) It was also difficult to stably feed the sheet in accordance with various specific gravities since the sheet feeding pressure varies depending on the specific gravities of sheets even when the sheets have the same size.
(3) The above problems (1) and (2) become further notable in the sheet feeding tray of a large capacity in which the number of stackable sheets is large.
There is a case in which it is necessary for a user to adjust or switch the sheet feeding pressure to obtain a required sheet feeding pressure.
The present invention has been made to solve the above problems inherent in the prior art, and therefore, an object of the present invention is to provide an apparatus for restraining a change in sheet feeding pressure in accordance with the size of a stacked sheet and a specific gravity of the sheet so that the paper can be stably fed.
The present invention is characterized by
sheet stacking means rotatably supported by an apparatus body and supporting a sheet;
sheet feeding means arranged on a leading end side in a sheet feeding-out direction of the sheet stacking means and feeding out the sheet supported by the sheet stacking means;
biasing means for pressing the sheet supported by the sheet stacking means against the sheet feeding means; and
load transmitting means for converting a load of the sheet applied to a rear end side in the sheet feeding-out direction of the sheet stacking means to a biasing force for biasing the leading end side of the sheet stacking means toward the sheet feeding means.